**13. Future perspectives**

Even if the very essence of crystal engineering is to produce functional materials, a large number of studies and papers in this field are still devoted solely to the structural aspects of the molecular assemblies and frameworks. The same is true with halogen bonding. The latter is understandable since establishing the whole concept of halogen bonding has required (and still requires) a considerable amount of work. Nevertheless, examples already exist of the utilization of halogen bonding in the production of functional materials. The role of halogen bonding has been investigated in the context of the inhibition of the human protein kinase CK2α.[74]It has also been used for selective recognition of halide anions and employed in host-guest systems.[23] There are examples of the use of halogen bonding for controlling the luminescent properties of Au2-Ag2 clusters and the birefringence properties of chains of square planar Au complexes[45,75]. There are also examples of the utilization of halogen bonding in catalysis.[76] In the future the number of these types of applications is expected to grow rapidly. All of this means that halogen bonding is in the process of being transformed from a strange solid-state phenomenon to a versatile tool in the hands of crystal engineers.
